Field of the Invention
The invention relates to gas analyzers, especially gas analyzers for mobile use, with a fuel cell to generate the electrical operating energy.
Description of the Related Art
Ion mobility spectrometers (IMS) and mass spectrometers (MS), which are also collectively referred to as gas analyzers hereinafter, are used in both the civil and the military sector for detecting dangerous or prohibited target substances on surfaces or in the ambient air. The ion mobility analyzers and mass analyzers used in the gas analyzers here can be coupled to gas chromatography devices or other analyzers, e.g. infrared spectrometers.
For the above-mentioned applications, ion mobility spectrometers are usually operated at ambient pressure, their distinguishing feature being a simple and compact design, which means they can be used in large numbers and as mobile detection devices. In an ion mobility spectrometer, the substances to be detected (target substances) and interfering and background substances are usually ionized by chemical ionization (APCI=Atmospheric Pressure Chemical Ionization). Electric fields then cause the ions to move in a drift gas, where they are separated on account of their ion mobility or the field-strength dependence of their ion mobility, and are detected in an ion detector. In contrast, in a mass spectrometer, for example a 2D or 3D Paul ion trap or a quadrupole filter, the ions of the substances to be detected are transferred into the vacuum and analyzed there according to their mass-to-charge ratio.
Due to the increasing terrorist threat the detection of explosives and chemical warfare agents has become very important not only in the military sector but also for homeland security (civil defense). The task, on the one hand, is to prevent their illegal import and attacks on transport means such as planes or ships. On the other hand, homeland security is increasingly also being extended to public buildings and means of transport at home. In addition to the threats posed by explosive substances and chemical warfare agents, there remains the task of detecting drugs as they are being smuggled across national borders. This particularly results in a greatly increasing demand for detection devices at airports, seaports and border control points, where the illegal or hazardous target substances are transported in items of baggage as well as in industrial containers. A further civil application consists in monitoring industrial sites and buildings for leaking chemical pollutants and use by the fire service for detecting harmful chemicals in traffic accidents, ship collisions or fires.
A special challenge is the detection of drugs and explosives in transport containers, for example in suitcases at airports, shipping containers at seaports or vehicles during vehicle spot-checks. The detection of modern explosives, and drugs also, is hampered by the fact that these substances have a very low vapor pressure and are also often sealed in transport containers. In most cases, this means direct detection in the ambient air is possible only if a large sample volume is taken and the substances from the sample are enriched. However, when the target substances are being packed, minimal traces contaminate the surfaces of the baggage items, the transport containers and the clothes and skin of the persons packing the substances. The target substances themselves are present as condensed vapors on the surface itself or on any particles adhering to the surface, but the vapor pressure they develop is too low to allow direct detection in the ambient air.
Owing to the low vapor pressure, the surfaces to be examined are usually wiped with a sampler (made of paper or a Teflon-coated glass-fiber fabric, for example), which causes condensed target substances and any particles carrying target substances to be removed from the surface and to adhere to the sampler. The sampler with the target substances is transferred into a desorption device of a gas analyzer, where it is heated in order to achieve a vapor pressure for the target substances which is sufficient for detection. It is also possible to press a heated probe directly onto the surface under examination and to pass the vapors released in the process to the gas analyzer.
The substances present in the gaseous phase due to desorption or the appropriate vapor pressure (target substances as well as interfering and background substances) can condense in the interior of the gas analyzer and interfere with subsequent measurements. A simple, but effective way of minimizing adsorption and memory effects consists in heating all surfaces which come into contact with the substances. For this type of operation, modules having such surfaces are preferably heated to temperatures above the ambient temperature. The operating temperatures are between 50 and 200 degrees Celsius (° C.). For example, the inlet region of a mass spectrometer for the detection of explosives is usually heated to a temperature between 120° C. and 200° C., and corresponding modules of an ion mobility spectrometer are operated at temperatures between 50° C. and 100° C.
With many gas analyzers, the substances present in the gaseous phase first pass into the interior of the gas analyzer via a permeable membrane, said membrane being flushed from the outside with a sample gas containing the substances. Using a membrane made of organic polymers such as silicone rubber has the advantage that most of the organic substances to be detected pass through the membrane better than interferents or water so that, with ion mobility spectrometers in particular, a disadvantageous input of moisture is reduced. Experience has shown that a membrane inlet requires heating in order to minimize delay and memory effects in the membrane material. The delay and memory effects also occur in the gas channels leading to the ion source and in the ion source itself, and therefore gas analyzers which have a direct gas inlet instead of a membrane inlet are also affected. Moreover, it is often necessary to protect the gas analyzers used against ingress of dust, rainwater and other foreign substances. Dust filters made of wire cloth or porous materials are usually used for this purpose. These materials have relatively large surfaces in contact with the gas, and therefore low-volatility substances are adsorbed there or condense to a considerable degree, especially at low ambient temperatures.
A further reason for being able to heat modules in gas analyzers consists in keeping modules at a constant operating temperature in order to minimize temperature-dependent parameter variations, such as the permeation rate of inlet membranes. A constant operating temperature requires either means of switching between heating and cooling, which makes it necessary to have equipment of great technical complexity, or the modules must be operated permanently at a temperature above the highest ambient temperature and thus be permanently heated.
Stationary gas analyzers are supplied with energy by connecting them to a fixed power supply, where possible. Gas analyzers for mobile use are powered by electrochemical cells (disposable or rechargeable batteries), in particular by electrochemical cells with a high energy density such as lithium-manganese or lithium-ion cells. The modules operated above ambient temperature are heated electrically, which is why the electrochemical cells used for the energy supply in these cases constitute a significant proportion of the volume and weight of mobile gas analyzers.
The technical development of smaller fuel cells (FC) with powers up to a few 10 W, which are used in mobile computers and communication equipment, is far advanced. These kinds of fuel cells can also be used for mobile gas analyzers. The publication US 2004/0120857 A1 (Smith et al.) discloses a network of sensors which detect target substances at different locations and transmit measurement data to a control unit. Said sensors can be mass spectrometers and ion mobility spectrometers, for example, and the energy supply for the sensors comes from electrochemical cells or fuel cells. For use in small mobile devices, mention must be made of direct methanol fuel cells (DMFC), which achieve energy densities that exceed those of commercial electrochemical cells several times over. The direct methanol fuel cells work at an operating temperature of between 60° C. and 130° C., and have an efficiency of up to 40%.
There are other types of fuel cells apart from the DMFC. Fuel cells with electrolytes of molten salts (molten carbonate fuel cell, MCFC) are operated with hydrogen, methane or coal gas at an operating temperature of around 650 degrees Celsius and have an efficiency of 48%. For toys and science kits, magnesium-air fuel cells (MAFC), which operate with magnesium as the fuel at an operating temperature of 55 degrees Celsius and have an efficiency of up to 90%, are commercially available. To operate with hydrogen and oxygen (also oxygen from the air), there is particularly the polymer electrolyte fuel cell, also known as proton exchange membrane fuel cell (PEMFC), with polymer membranes, which operates at efficiencies of between 35% and 60% at operating temperatures of between 90 and 120 degrees Celsius. The phosphoric acid fuel cell (PAFC, 38%, 200° C.), likewise with a polymer membrane, is operated with hydrogen and air, but pure gases are not required. The alkaline fuel cell (AFC, over 60%, <80° C.) is also operated with hydrogen and air, but the air must not contain any CO2. The solid oxide fuel cell (SOFC) uses carbon and oxygen at an operating temperature of 800-1000° C. and an efficiency of 47%.
The objective of the invention is to provide gas analyzers which can operate for as long as possible without a fixed power supply or in mobile use, and which have compact dimensions at the same time.